Introduction of a crosslinked structure into a resin to enhance cohesive property of the resin and to improve physical properties such as heat resistance and mechanical strength is a generally adopted practice for the purpose of improvement of properties of the resin for commercial applications. However, the resin to which a crosslinked structure has been introduced forms gel easily to cause a problem for molding. Because of this reason, the crosslinking process is generally performed after the molding has been finished. The crosslinking process is performed, for example, by application of radiation or by utilizing a reaction with moisture in the air. These methods have problems that an article not yet crosslinked must be handled after the molding and that the cost of processing is rather high. Recycling of waste plastic article and waste parts formed in the punching process and in the molding process, is progressively required for the environmental protection and for the saving of materials. When a crosslinked structure is introduced in a resin, recycling of the crosslinked material is very difficult to cause a problem on the environmental protection and on the saving of materials.
Various methods have been proposed for solving these problems. In many of these methods, the crosslinking in a resin is dissociated during the molding process and the crosslinking is formed while the resin is cooled and solidified after the molding. In the methods proposed in U.S. Pat. Nos. 3,264,272, 3,267,083, 3,789,035 and 3,997,487, carboxylic acid groups are introduced into a resin and a pseudo-crosslinking is formed in the resin by forming a salt of a metal with the carboxylic acid group. In the methods proposed in U.S. Pat. Nos. 3,328,367 and 3,471,460, carboxylic acid groups are introduced into a resin and a pseudo-crosslinking is formed by forming a salt of an organic diamine with the carboxylic acid group. Other methods, such as the method utilizing Dieis-Alder reaction [J. P. Kennedy and K. F. Castner, Journal of Polymer Science, Polymer Chemistry Edition, Volume 17, Page 2055 (1979) and J. P. Kennedy and G. M. Carlson, ibid., Volume 21, Page 2973 (1983)] and the method utilizing the reaction of nitroso group (U.S. Pat. No. 3,872,057), were also proposed.
Still other methods, such as the method utilizing the reaction of an acid anhydride group with an alcohol or an amine [U.S. Pat. Nos. 3,299,184 and 3,678,016 and J. C. Decroix et al., Journal of Polymer Science, Polymer Symposium, Volume 52, Page 299 (1975)] and the method utilizing the ester exchange reaction of a resin containing hydroxyl group and a diester [D. S. Camplell, Chemistry and Industry (London), Volume 7, Page 279 (1974)], were also proposed.
However, the resins crosslinked with the pseudo-crosslinking have insufficient heat resistance because the crosslinking is easily dissociated with increase of temperature. The resin utilizing Dieis-Alder reaction has problems that the commercial production is difficult and that the crosslinking is not completely dissociated even at a high temperature to cause difficulty in molding. The resin utilizing the reaction of an acid anhydride and an alcohol or an amine or the reaction of ester exchange is not suitable for the commercial production because the crosslinking reaction during the cooling and solidifying of the resin is slow or the crosslinking reaction and the dissociation reaction cannot be controlled even though this method is the closest to the commercial production among the methods proposed.